Conventionally, a relief valve is known which is arranged between an engine and an oil pump circulating a working fluid to lubricated members a the engine, and adjusts a discharge pressure of the oil pump (for example, refer to Patent documents 1 and 2). The relief valve includes a housing provided with an internal flow passage in which the working fluid flows and a relief port discharging the working fluid from the internal flow passage, and a valve body reciprocating inside the housing. A pressure of the working fluid acts on an upper surface of the valve body and a biasing force of a spring acts on the valve body in a direction opposing the pressure. In a case where the pressure exceeds the biasing force of the spring, the relief port is opened and the working fluid is discharged to an upstream-side relative to the oil pump, thereby reducing the discharge pressure of the oil pump.
A relief valve of Patent document 1 includes a support portion which is capable of reciprocating while supporting an end portion, at a side opposite to a valve body, of a spring. The relief valve supplies a working fluid, which is discharged from an oil pump, to a back surface of the support portion via a three-way valve. During a normal operation of an engine, the three-way valve is controlled to supply the working fluid to the back surface of the support portion, and thus the spring is compressed, thereby increasing a relief pressure. On the other hand, during a worm-up of the engine, the three-way valve is controlled to discharge the working fluid existing at the back surface of the support portion, and thus the spring is expanded, thereby reducing the relief pressure. It is described that the pressure of the working fluid flowing to the engine is thereby reduced and the worm-up is facilitated.
Patent document 2 discloses a technique that a switch valve is provided between, an oil pump and a relief valve and that the switch valve is operated to open to increase a pressure-receiving area of a valve body, thereby assisting relief valve to open. That is, it is configured such that, by increasing and decreasing the pressure-receiving area of the valve body in a state where a biasing force of a spring is maintained constant, a high-pressure relief mode and a low-pressure relief mode can be selected. In addition, when operating the switch valve to open and close, an electromagnetic valve provided between the oil pump and the switch valve is controlled to turn ON/OFF, thereby supplying/discharging a working fluid, relative to a spool of the switch valve.